An increasingly popular monitoring technique in medium and high voltage plant is detection of partial discharges (PDs). It has been known for a long time that many problems start with a PD in generator windings, transformers, switchgear or on cabling. But it is the development of electronic systems that have increased the popularity and effectiveness of the technique.rnPDs are a type of localised discharge and result from transient gaseous ionisa-tion in an insulation system when the voltage stress exceeds a critical value. They occur at poor electrical connections, inside windings and on failing bushings. They generate electromag' netic radiation over a very wide spectrum, send high frequency currents along cabling and also generate acoustic waves. These radiate in all directions with a frequency and magnitude dependent on the nature of the discharge.
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